The present invention relates to ferrous compositions which may be used for roll shells, a component of steel caster rolls which are used to strip cast aluminum alloys. The shells have excellent resistance to heat checking at higher shrink-fit stresses between the shells and the water cooled cores.
Molten aluminum at about 675.degree. C. is directly cast to strip between two water cooled rolls each composed of a roll shell (hoop or sleeve) which has been shrink-fitted on a water cooled core. The rolls arc driven rotationally in opposite directions and the distance between them determines the thickness of the cast strip. The cast strip solidifies in contact with the rolls. The main function of the shells is to contain and extract heat from the molten aluminum to control the solidification and provide a good aluminum cast surface.
The shells must have excellent mechanical properties such as high strength and toughness since the rolls arc subjected to stresses caused by separating forces and roll drive forces during casting. The shells will also have to withstand the mechanical stress due to the shrink-fit between the shells and the water cooled cores. Additionally, the shell surfaces will continuously expand and contract during the cyclic temperatures developed during casting. This cyclic stress developed on the surfaces will contribute to the initiation and propagation of any cracks or defects formed during casting. Therefore, the rolls must also possess good resistance to thermal fatigue.
Thermal fatigue is caused by any variation in temperature which generates a change in dimension. If a material is heated homogeneously, the uniform temperature change will bring a change in volume but no stress. However, a temperature gradient in the material causes stress to occur in relation to the thermal gradient. When the shell surface contacts the molten aluminum, the surface temperature rapidly increases while the bulk of the shell is cooled by thermal transfer from the water cooled core. The stress levels reached will exceed the compressive yield strength of the steel and result in plastic deformation of the shell surface while in contact with the molten aluminum. When that portion of the shell moves away from the molten aluminum, it rapidly decreases in temperature and will contract causing high surface tensile stresses. Numerous cycles of this type will eventually cause mechanical fatigue and cracking when the ductility is exhausted. After a number of hours of casting, the surfaces of any roll shells will develop heat checking patterns or surface cracks which grow deeper into the shells and eventually mark the cast strip. These defects may also cause complete failure of the roll shells if allowed to grow excessively large. Periodic reconditioning of the shells is to be expected. Typically, reconditioning will remove up to about 0.15 inches (3.8 mm) of the shell thickness.
Resistance to heat checking and cracking is generally associated with a low coefficient of thermal expansion, high thermal conductivity, high elevated temperature yield strength, high elevated temperature ductility and a low modulus of elasticity. This combination of properties is difficult to attain and attempts to improve one of these properties has usually resulted in a sacrifice in one or more of the other properties.
The shells must also have good mechanical properties at room temperature because of the many stresses introduced during machining and reconditioning. Good ductility and toughness are very important to avoid brittleness during grinding and handling which could cause further cracking.
U.S. Pat. No. 4,409,027 (assigned to Armco Inc.) used a ferritic alloy composition for the roll shell which consisted essentially of, in weight %, about 0.53 to about 0.58% C, about 0.4 to about 1% Mn, 0.1 to 0.2% Si, about 0.02% max P, about 0.02% max S, about 0.45 to about 0.55% Ni, 1.5 to 3.0% Cr, 0.8 to 1.2% Mo, 0.3 to 0.5% V and balance essentially iron.
Stresses which are greater than the yield strength and tensile in nature will produce heat checks or cracks by a thermal fatigue mechanism when cyclic yielding or plastic flow occurs. A hysteresis loop can be plotted representing the accumulation of plastic damage during each cycle for the circumferential stresses perpendicular to the longitudinal cracks in the roll shell surface. In prior an chromium-molybdenum steels, the number of cycles to failure is about 10.sup.4 if the extent of plastic deformation per cycle is about 0.001 inch per inch or slightly less. High carbon levels were required to provide hardness, strength and resistance to localized softening which resulted from the distribution of carbon in the microstructure. Molybdenum and vanadium were increased to form carbides which increased the elevated temperature strength. By increasing the elevated temperature yield strength by 50-100%, the service life was increased three-fold if the elevated temperature ductility was maintained. This patent found that increasing the elevated temperature yield strength more than compensated for the loss in thermal expansion, modulus of elasticity and conductivity when improving the resistance to checking.
In the equation set forth in U.S. Pat. No. 4,409,027 for total thermal strain [thermal strain (.epsilon..sub.t)=.alpha..DELTA.T where .alpha. is the coefficient of thermal expansion and .DELTA.T is 1150.degree. F. which represents the difference between the maximum roll surface at 1250.degree. F. and the minimum roll surface temperature of 100.degree. F.], the total strain is assumed to be the sum of elastic and plastic components: EQU .epsilon..sub.t =.epsilon..sub.elastic +.epsilon..sub.plastic =.sigma.y/E +.epsilon..sub.plastic
where .sigma.y is the yield strength in tension and E is the elastic modulus
Thus, the elastic component of the strain is represented by the yield strength in tension divided by the elastic modulus. In the case of a steel having a yield strength of 200,000 psi at room temperature, .epsilon..sub.elastic =.epsilon.y/E=200,000/30.8.times.10.sup.6 psi-6.49.times.10.sup.-3 in/in. If the yield strength is decreased by 50% at elevated temperatures, such as 1250.degree. F., the .epsilon..sub.elastic =.epsilon.y/E=100,000/24.times.10.sup.6 psi=4.16.times.10.sup.-3 in/in.
Where the total strain is equal to or greater than twice the elastic strain, to account for the compression and tension portion of elastic reaction, then plastic flow is possible in both the compression and tension ends of the cycle. With a 50% decrease in yield strength to 100,000 psi, 2.times..epsilon..sub.elastic =2.times.4.16.times.10.sup.-3 =8.32.times.10.sup.-3 in/in. The plastic component then becomes .epsilon..sub.plastic =.epsilon..sub.t -2.times..epsilon..sub.elastic =8.0.times.10.sup.-4 in/in.
Consequently, a plastic flow of about 0.001 inch per inch per cycle is possible, which would indicate a potential exhaustion of plasticity and failure in 10.sup.4 to 10.sup.5 cycles.
The calculations are believed to support the belief that the high elevated temperature yield strength of the steel causes a much greater percentage of the thermal expansion and contraction to occur in the elastic region. This minimizes the plastic reaction and results in much greater resistance to heat checking. Maintenance of the high ductility of the steel at a higher yield strength insures a retarded crack growth rate once heat checking does occur.
While the roll shell of this patent had good elevated temperature strength, the material did not have good toughness and ductility at room temperature which is required for a higher shrink-fit to minimize slippage between the shell and core. Improving the toughness would also allow the material to withstand the handling required during grinding to remove heat checks and resist the propagation of cracks.
A standard alloy used for roll caster shells referenced in U.S. Pat. No. 4,409,027 comprised 0.53-0.58% C, 0.45-0.65% Mn, 0.2-0.3% Si, 0.4-0.5% Ni, 1-1.2% Cr, 0.45-0.55% Mo, 0.1-0.15% V, 0.02% max P, 0.02% max S and balance essentially Fe.
Another alloy referenced in U.S. Pat. No. 4,409,027 as an Al die casting alloy had 0.3-0.4% C., 0.2-0.4% Mn, 0.8-1.2% Si, 4.75-5.5% Cr, 1.25-1.75% Mo, 0.8-1.2% V and balance Fe. It was discussed as being expensive and difficult ti process.
U.S. Pat. No. 4,802,528 (assigned to Chavanne-Ketin) produced forged casings for continuous casting aluminum from an alloy steel having 0.3-0.36% C, 0.3-0.6% Mn, 0.15-0.45% Si, less than 0.4% Ni, 2.8-3.4% Cr, 0.85-1.25% Mo, 0.1-0.3% V, 0.02max P, 0.02% max S, 0.3% max Cu, and balance essentially Fe. This alloy reduced the carbon content from previous alloys to improve ductility and toughness. While sacrificing some thermal conductivity, the grade of steel is stated to have a surprising improvement in resistance to thermal fatigue cracking. This patent studied the mechanical and thermal cycles of the roll during shrink-fitting and casting of aluminum. The roll shell has mechanical stress related to roll production which produces circumferential tensile stress and longitudinal stress and it has operating stress from torsion due to the driving torque and bending stress due to separating forces during casting. The thermal stresses relate to the difference in temperature between the inner cooled core of the roll and the temperature of the molten aluminum which cause the roll shell to exceed the elastic limit of the steel which causes plastic deformation. The cooling of the roll causes the deformation to start to disappear but the shell can not return to its original position because of the plastic deformation in compression. The return to the low temperature will cause the elastic limit to be exceeded in tension and result in plastic deformation. This cycle of deformation of thermal origin will cause fatigue of the surface and result in the initiation and subsequent propagation of microcracks. One of the essential properties demanded of the roll shell is the resistance to thermal fatigue.
An alloy referenced in U.S. Pat. No. 4,802,528 used for roll caster shells had 0.53-0.57% C, 0.9-1.3% Cr, 0.4-0.6% Mo, 0.1-0.2% V, 0.7-0.9% Mn, 0.4-0.7% Ni and 0.2-0.4% Si, 0.02% max P, 0.02% max S and balance essentially Fe.
Another alloy referenced in U.S. Pat. No. 4,802,528 used for roll caster shells had 0.53-0.58% C, 1.5-3% Cr, 0.8-1.2% Mo, 0.3-0.5% V, 0.4-1.0% Mn, 0.45-0.55% and 0.1-0.2% Si, 0.02% max P, 0.02% max S and balance essentially Fe.
U.S. Pat. No. 4,861,549 (assigned to National Forge Company) disclosed a ferritic steel preferably containing 0.45-0.49% C, 1.20-1.50% Ni, 0.90-1.00% Mn, 1.20-1.45% Cr, 0.010% max P, 0.80-1.00% Mo, 0.002% max S, 0.15-0.20% V, 0.15-0.35% Si, up to 0.08% rare earth metals and balance Fe. The steel was designed for roll shells to cast aluminum sheets. The steel was stated to have good resistance to heat checking and cracking when subjected to extreme heat stresses in repeated thermal cycles. Rare earth elements were added to avoid temper embrittlement.
Prior attempts to provide good elevated temperature yield strength for reducing heat checking and craze cracking have also increased the room temperature yield strength. This has an adverse effect on the ductility and shrink-fit process. High carbon was used to provide solution strengthening for elevated temperature strength but this made the steel brittle at room temperature and had an adverse effect on toughness.
Even though some improvements in service life were obtained for the rolls used to strip cast aluminum alloys, there is still a need for further improvement in order to increase production and improve the quality of the aluminum strip. There is also a need to increase the shrink-fit between the shell and the core of the roll without adding excessively to stresses in the shell. This allows for less slippage of the shell over the core during casting. Prior roll shell alloys had chemistries balanced to provide good high temperature properties and were less concerned with room temperature properties which are critical for shrink-fit and machining properties.